Exploring a New Family of (Phosphinochalcogenoyl)Europocenes for Magneto-Optical Thermometry

The development of novel materials based on lanthanide metallocenes remains a valuable endeavour, due to the synthetic accessibility, compositional tunability, and impressive magnetic and luminescent properties of this class of well-studied compounds. Europium(II) complexes are particularly interesting in this regard, given the relative stability, strong paramagnetism, and characteristic Laporte-allowed 5d→4f emission of this ion. In this study, we tap into the powerful class of phosphinocyclopentadienyl ligands to develop a novel family of potassium (bis(dimethylamino)phosphinochalcogenoyl)cyclopentadienides K[C₅Me₄(PCh(NMe₂)₂] of sulfur, selenium, and tellurium, and explore their coordination chemistry with Eu^II. These ligands readily form homoleptic europocenes where the pendant phosphine chalcogenide arms also coordinate the europium center, and these are characterized via single-crystal X-ray diffraction, absorbance, and photoluminescence spectroscopy. Characterization via magnetic circular dichroism (MCD) spectroscopy reveals that these complexes act as extremely sensitive probes for MCD-based magneto-optical thermometry, reaching relative thermal sensitivities up to 170 % K^-1 at cryogenic temperatures, with temperature uncertainties as low as 0.003 K. These complexes represent the first examples of organometallic MCD thermometers, and their remarkable properties and interesting structures highlight the promise of this new class of ligands and the utility of the Eu^II ion in lanthanide-based functional materials research.